There has become an increasing trend during the past two decades or so to render devices free of electrical supply feeders. To this end, hand-held portable appliances powered by internal batteries have become increasingly popular and this trend has become more manifest as the technology of rechargeable cells has improved. In order to ensure compatibility between the batteries and appliances manufacturers, standardization is widely effective in the batteries arena. As such, the batteries are manufactured in defined output voltages (1.5, 4.5, 9 Volts, for example), as well as defined "standard" mechanical enclosures (usually defined by letters "A", "AA", "B" sizes etc.). Likewise, corresponding battery compartments are provided in electronic appliances for accommodating batteries of specific size
Essentially, there exist two types of batteries: disposable or primary batteries intended for one-time use; and rechargeable or secondary batteries which can be recharged many time by connecting to an appropriate charger. Rechargeable batteries, although initially more expensive than their primary counterparts, quickly pay for themselves owing to their multiple use and are more environmentally "friendly" because they are disposed of much less frequently. In many cases they may even be safer for the end user.
As against these benefits, rechargeable batteries are often less convenient than disposable cells which are always on tap, fully charged and ready for use. In contrast to this, rechargeable cells must be regularly recharged, requiring opening of the appliance or its battery compartment, removal of the battery and its connection to the charger. Such a process is frequently cumbersome and time-consuming. Furthermore, in order to keep the appliance continuously ready-for-use, at least one standby battery must be maintained fully charged: thus increasing the running cost as well as adding to the inconvenience.
The drawback associated with removal of the battery from the appliance has been addressed in the prior art: principally by means of the provision of non-contact battery chargers allowing the battery to be recharged in situ by electromagnetic induction. Thus, U.S. Pat. No. 5,600,225 to Goto concerns the charging of rechargeable batteries installed in radio communication devices such as radiotelephones by means of devices which do not require connection to the radiotelephone. The particular aspect to which Goto relates to improving the charging efficiency whilst also allowing easy removal of the radio communication device from the charger, should it be necessary to make or receive a call during charging.
U.S. Pat. No. 4,873,677 to Sakamoto et al. discloses an apparatus for charging a rechargeable battery within an electronic device, particularly a wrist watch. A power source provides direct current which is alternated in direction through a pair of primary coils for inducing alternating current in a secondary coil of the electronic device. A battery within the electronic device is trickle charged via a half wave rectifier connected to the secondary coil.
Other examples of prior art references to non-contact battery chargers include U.S. Pat. No. 4,031,449 to Trombly which discloses a solid state electronic battery charger which is electromagnetically coupled to one or more batteries. U.S. Pat. No. 3,938,018 to Dahl and U.S. Pat. No. 5,550,452 to Shirai et al. both disclose induction charging systems, and U.S. Pat. No. 5,396,538 to Hong discloses a contactless charging system of a radio telephone comprising a portable device and a base unit, for charging a battery of the portable device by induction voltage of an electromagnetic field formed by inductive coupling. There are many other prior art references relating to the contactless recharging of electrical vehicle batteries.
It is thus apparent that the drawback relating to the need for a rechargeable battery which may be recharged within an appliance is addressed in the prior art. However, all of the systems described in the prior art employ additional circuitry which, in minimum form, includes a coil for inductive coupling with an external source and suitable rectification for allowing trickle charging of the battery. This, in turn, requires that the appliances for use with such systems must themselves be custom designed in order to accommodate the additional circuitry: thus adding to the cost thereof.
Furthermore, the charger must be mechanically designed in tandem with the appliance in order to increase the mutual induction coefficient between the coils in the charger and in the appliance. The specific mechanical structure which thus results prohibits the use of the charger or the appliance for applications other than those designed for.
The additional circuitry inside the appliance is "built-in" and cannot be removed, thus precluding the usage of standard (non-rechargeable) batteries. Such modification to the appliances may be acceptable when use of the enclosed rechargeable battery is obligatory; such as when access to the battery compartment is so bothersome that the replacement of primary, non-rechargeable, batteries is hardly viable.
In fact, there exist many occasions when access to the battery compartment allowing replacement of the battery is feasible and only mildly inconvenient, even though the use of rechargeable batteries is desirable if only because of their economic advantages. If, in these circumstances, it is decided to employ rechargeable batteries, then it is clearly frustrating to be compelled to removed the battery for charging with the attendant inconvenience that this implies.
Moreover, portable electronic appliances lose their portability once they are constrained to be used with rechargeable batteries. Often, such appliances are used remote from any source of electricity supply: such as when travelling, camping out, and the like. As noted above, this requires the provision of at least one fully charged standby battery which can be substituted for an expired battery. However, there is a limit to how many standby rechargeable batteries can conveniently be maintained available and therefore portable appliances must remain capable of being powered by readily available, relatively inexpensive, secondary batteries.
Yet a further consideration relating to portable appliances apart from the inconvenience of constant battery replacement is the cost of new batteries. Portable appliances are often used in an area having easy access to a main electricity supply socket and it is therefore known to allow such appliances to be powered by the main electricity supply usually via an adapter. However, this approach suffers from the drawback that the appliance becomes tied to the electricity supply socket and thereby loses some of its portability. Furthermore, in order to provide the option to save battery power even when the appliance is transported, the adapter must also be transported or a spare must be maintained at each principal location where use of the appliance is intended. This is inconvenient or costly.